Why it's so hard to call the end of a drought

Lack of rain has put many parts of the world in drought conditions. But diminished precipitation is only a part of the story when it comes to droughts.

The Almaden Reservoir in San Jose, Calif., on Jan. 28, 2014.
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The Almaden Reservoir in San Jose, Calif., on Jan. 28, 2014.

"It's not a single event," said Lynn Wilson, executive director of environmental research firm SeaTrust Institute and its head delegate to the United Nations. "Drought has longer-term effects on places than just the time the water isn't available. And people don't see them until it's too late, and it takes a long time to recover from them, even when they end."

And therein lies some of the difficulty about droughts. Unlike a hurricane, tornado or snowstorm, the end of droughts are just as hard to predict as their starting points.

Pinning down an end point

A common misperception about droughts is that the effects end with rainfall. That depends, experts say. While it certainly helps to have rain, it takes a lot of rainfall over a long time to end droughts. What's really needed is prolonged rain that makes soil wet at lower levels to keep it moist for plants and crops.

And cooler temperatures don't end droughts. They reduce the evaporation of water from the soil, but if a large amount of rainfall doesn't happen over time, a drought will continue.

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Another myth is that hot and humid weather hurts plants in a drought. Quite the contrary, experts say. If weather is breezy and sunny with low humidity, water evaporates more quickly from plants and soil to make the drought worse.

Soil actually holds its moisture well in humid weather, even if the temperatures are high. Also, morning dew is good for plants in humid weather as well.

'Insidious act of nature'

The National Drought Mitigation Center states that a drought is "an insidious hazard of nature." But in basic terms, drought comes from dry weather and lack of rainfall over an extended period of time, usually a season or more.

It results in shortages of water for people, crops and livestock, and an overall negative effect on the environment.

Droughts can be characterized as short-term—lasting six months—or longer, such as California's current drought, which is now in its third year and appears headed for a fourth.

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But a lack of rain, even under that definition, doesn't mean a drought is in effect. Analysts and experts must look at a host of figures such as annual rainfall, soil moisture, soil runoff and evaporation, as well as everyday water use, to determine if a drought is in place.

For instance, if there's a lack of rainfall in Indonesia for two weeks, that could be the start of a drought, because the country has a tropical climate and usually gets an average yearly rain fall of 106 inches. But a two-week lack of rain in a desert country such as Saudi Arabia would not be thought of as a drought with its annual rainfall at 2 inches per year.

Measuring a drought

To determine if a drought has begun, and to keep track of it, one of the most widely used methods, at least in the U.S., is the Palmer Drought Severity Index or the PDSI.

Developed by the U.S. Weather Bureau in 1965, the PDSI combines temperature, precipitation, evaporation, transpiration, soil runoff and soil recharge data for a given region to produce a single negative number representing conditions. The index serves as an estimate of soil moisture deficiency, which ultimately equals a drought's severity.

Here are how the ratings break down:

  • Abnormally dry (category D0, corresponding to a PDSI between -1.0 and -1.9)
  • Moderate drought (D1, PDSI between -2.0 and -2.9)
  • Severe drought (D2, PDSI between -3.0 and -3.9)
  • Extreme drought (D3, PDSI between -4.0 and -4.9)
  • Exceptional drought (D4, PDSI between -5.0 and -5.9)

Currently, more than 80 percent of California is in extreme drought, or has a PDSI rating between -4.0 and -4.9. But around 58 percent of the state is in exceptional drought—the highest level of measurement.

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As to when a drought officially ends, that comes down to how much moisture or rainfall has occurred to bring conditions like soil wetness and water runoff back to normal or above normal levels, according to the National Climatic Data Center. That could take weeks, months or even years after the rainfall begins.

Drought effects depend on where you are

The effects of drought vary from country to country, but the results are mostly disastrous. Like many drought-ridden countries, Kenya has imported several crops to offset the effects from loss of its own agriculture commodities. Kenya's drought has forced coffee plantations there to move to higher ground in order to find water sources. And many Kenyan families in rural areas have moved to cities to live with relatives who can help feed them.

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In Australia, livestock and agriculture production have declined. The country is also suffering from wildfire outbreaks due to the lack of rain. Eighty percent of Queensland, the third most populous state in Australia, is in drought.

In Colombia, the government has established new water-rationing regulations in the coastal and Andean regions. The government there said the worst of the drought is "yet to come."

Brazil, Guatemala, Pakistan, Somalia, Australia and China have suffered from lack of drinking water as well as crop and livestock losses.

Meanwhile, California's drought is expected to cost the state $2.2 billion and put some 17,000 agricultural workers out of a job in 2014, according to a study from the University of California, Davis.

Something else that can't be ignored is the human cost of drought. The World Economic Forum reports that since 1900, global droughts have affected two billion people, leading to more than 11 million deaths.

—By CNBC's Mark Koba.